Download National 5 Biology Unit 3 Life on Earth Summary Notes

National 5 Biology
Unit 3 Life on Earth
Summary Notes
Adaptation, natural selection and the evolution of species
A mutation is a random change to genetic material. In the mutation shown below, a
section of DNA, three base pairs long, has been lost.
DNA contains genes that carry instructions for the manufacture of a protein. If a
mutation occurs in a gene that results in a change to the sequence of DNA bases, then
the structure of the protein that is made may also be altered. This could alter an
individual's phenotype.
A mutation can be neutral and have no effect or can have an advantageous or
disadvantageous effect on an individual that possesses it. Mutations are the only source
of new alleles.
Mutation
Effect on individual's ability to survive and reproduce
Advantageous
Increased
Disadvantageous Decreased
Neutral
No change
Some people carry a mutated version of the gene that codes for haemoglobin. The
protein that is coded for by the mutant allele doesn’t function correctly and can
distort the shape of red blood cells.
Three circular red blood cells
If a person has two copies of the mutant allele in their body cells then they will have
the genetic condition sickle cell anaemia. The sickle shaped red blood cells can block
blood vessels and have a much shorter life span than normal red blood cells.
Mutations happen spontaneously. The rate of mutation can be increased by
environmental factors such as UV radiation, X-rays, gamma rays and certain types of
chemicals such as bromine.
A red blood cell with a distorted 'sickle' shape.
Evolution by natural selection
Evolution can be defined as the change in the frequency of a phenotype in a population
over many generations.
Variation
Although members of a species have many features in common they are not identical to
each other. Variation occurs as a result of mutations that create new alleles of genes.
Mutations can be inherited if they occur in DNA that is passed on to the next
generation. In populations that produce offspring by sexual reproduction, variation also
occurs as a result of individuals receiving different combinations of alleles from their
parents.
Variation in a population allows a population to evolve over time, and this may occur as a
result of changing environmental conditions.
Natural selection
Individuals produce more offspring than their environment can support, and some die
because of factors such as predation, food shortage or disease. These factors are
known as environmental selection pressures and they determine which individuals will do
best at surviving and reproducing.
Some individuals are better adapted to their environment than others as a result of
characteristics they have inherited from their parents. The genes that produce these
characteristics give them a selective advantage. They may have a better ability to
avoid predators, compete for food, or resist disease.
These individuals are therefore better able to survive and reproduce than less welladapted individuals. They pass on the versions of genes that produce their adaptations.
Predation, competition and disease are examples of selection pressures.
Example of natural selection in action
Industrial melanism in the peppered moth
In the peppered moth population of Great Britain there is variation in the colour of the
moth’s bodies. Some moths have a light body; some have a dark, or melanic body. Body
colour is an inherited characteristic.
Three moths camouflaged against different background colours and textures
Before the industrial revolution, light moths had a selective advantage. Because they
were well camouflaged against the lichen-covered trees, they were better able to avoid
predation.
During the nineteenth century pollution killed off some of the lichens and soot deposits
caused the tree bark to appear darker. Light coloured moths were no longer
camouflaged and were more likely to be eaten by birds. The dark moths were now
better camouflaged.
As a result, dark moths had a greater chance of reproducing and passing on the alleles
that made them dark. This led to a gradual increase in the proportion of dark moths
until light moths became very rare in industrial areas. The moth populations in these
areas had evolved.
This change from predominantly light to dark forms in industrial areas is called
industrial melanism.
Note that this change was not due to pollution making the moths darker. The dark
variety had always existed in small numbers, but only had a selective advantage when
the selection pressure changed.
Speciation
A species is a group of organisms able to interbreed and produce fertile offspring.
Speciation is the formation of two or more species from the original one.
As long as the individuals in a population have the opportunity to interbreed and
combine genes, they remain one species. A population of one species can only evolve into
more than one species if groups within the population become isolated from each other.
The diagram illustrates what could happen to an animal population, which is separated
into two isolated groups by a geographical feature. In addition to geographical
separation, species can also be isolated by ecological or reproductive barriers.
Different mutations will occur in the isolated groups (because mutations occur at
random).
If the environmental conditions differ in the two locations then the individuals in the
two isolated groups will experience different selection pressures. Different
phenotypes will be selected for in the two groups.
Over many generations the isolated groups will evolve to be different from each other.
Eventually the groups may become so different that if they come together again they
are no longer able to interbreed and produce fertile offspring and so are now separate
species.
Speciation in honeycreepers
The islands of Hawaii were colonised by a single species of finch-like birds. Eventually
populations were isolated from each other on separate islands.
The diagram shows how they evolved into the modern honeycreeper species with beak
shapes which adapt them for different feeding methods.
Biodiversity and the distribution of life
Biodiversity is the variety of species present in the community of an ecosystem. It is
affected by abiotic (non-living environmental) and biotic (interaction associated with
living things) factors.
Ecology definitions
Ecology is the study of the interaction between organisms and their environment.
Term
Definition
Biodiversity The variety of species present in the community of an ecosystem
Biome
Biomes are the various regions of our planet that are distinguished by
their climate and distinctive group of plants (flora).
Community All the organisms present in an ecosystem
Term
Ecosystem
Definition
All the organisms living in a particular area and the non-living components
that the organisms interact with
Habitat
The place where an organism lives
Niche
The role an organism plays within its community
Population
All the organisms of one species found in a particular area
Species
A group of organisms capable of breeding with each other to produce
fertile offspring
Biomes
The area of the Earth and its atmosphere where living organisms are found is called
the biosphere.
The biosphere can be divided into large areas called biomes that are distinguished by
their climate and distinctive group of plants (flora) and animals (fauna) living there.
Terrestrial biomes are found on the land and aquatic biomes include freshwater and
marine environments.
The map shows the Earth's major terrestrial biomes.
Distribution of biomes
The type of biome found in a particular region of the planet can depend on the average
annual rainfall and temperature.
The effects of abiotic and biotic factors on biodiversity
Abiotic factors
Abiotic factors are non-living environmental factors that can affect the organisms in
ecosystems.
Examples of abiotic factors are:

light intensity

soil moisture level

soil pH

temperature
The values of the abiotic factors in an ecosystem affect the range of species that are
found. This is because the individuals in each species are adapted to occupy particular
niches.
Biotic factors
Biotic factors are interactions associated with living organisms.
Examples of biotic factors are:

competition for environmental resources

grazing

parasitism (where one organism, the parasite, benefits from harming another
organism, the host)

predation
The effects of grazing on biodiversity
Grazing is carried out by animals such as rabbits and sheep that feed on a variety of
plant species. At low grazing intensities the biodiversity of a grassland is low because a
few species of plants are able to outcompete the others and dominate the ecosystem.
As the grazing intensity increases the biodiversity increases as the dominant plant
species are kept in check by grazers and the weaker competitors are able to grow as
well. At very high grazing intensities the biodiversity decreases because only plants
with adaptations to resist the effects of grazing are able to survive.
Human influence on biodiversity

The burning of fossil fuels leads to an increase in sulphur dioxide in the
environment which makes acid rain. Acid rain has devastating consequences for
biodiversity as many plants and animal species cannot survive these conditions.
As the pH of rain falls (becomes more acidic) biodiversity decreases.

Sewage must be treated before it is released into the environment. If
untreated sewage gets into rivers it provides food for bacteria, which
reproduce and increase in number. As their numbers increase they use up the
oxygen supply in the water. This results in a decrease in species diversity since
only species that are adapted to living in water with low oxygen concentrations
will survive.

Humans clearing away vast areas of natural forest for their own benefit. This
can result in habitat destruction, a reduction in soil fertility and poor soil
structure leading to a decrease in biodiversity.

Human activities resulting in large areas of land covered by desert, which is a
dry habitat for species leading to a decrease in biodiversity in these areas.
Niche
An organism's ecological niche is the role that it plays within the community of an
ecosystem.
The ecological niche of an organism is a complete description of:

The range of abiotic factors it can tolerate.

The resources in the ecosystem that it is able to make use of (eg the soil
nutrients available to it).

Its interactions with other organisms (biotic factors).
Energy transfer in ecosystems
All organisms require an energy supply for cellular activities such as protein synthesis
and cell division. This allows organisms to grow. Some of the energy is stored in the new
material that is made by growing organisms and this is called biomass.
Organisms release energy from their food by the process of respiration. In most
ecosystems on the planet the ultimate source of energy is the sun.
Producers make their own food using energy from an abiotic source. For example plants
carry out photosynthesis to make food using light energy from the sun. Consumers get
food from a biotic source by eating the biomass of producers or other consumers.
The feeding relationships that exist in an ecosystem can be shown by a food chain. The
arrows represent the flow of energy and mean 'gets eaten by'.
Not all the energy is passed from one level of the food chain to the next. About 90 per
cent of energy may be lost as heat (released during respiration), through movement, or
in materials that the consumer does not digest.
The energy stored in undigested materials can be transferred to decomposers.
Decomposers are organisms such as bacteria and fungi that can obtain energy by
breaking down dead organisms.
Pyramids of biomass
A pyramid of biomass shows the total dry mass of organisms at each level in the food
chain of an ecosystem.
Pyramids of biomass usually show that the producers have the highest biomass of all
the organisms in the ecosystem and the biomass then decreases at each level in the
food chain.
The biomass decreases because energy is lost at each level in the food chain so less
energy is available to be stored as biomass in the bodies of the consumers.
Pyramids of energy
A pyramid of energy shows the total quantity of available energy stored in the biomass
of organisms at each level in the food chain of an ecosystem per year. This can be
estimated by burning samples of organisms and measuring the heat released.
Pyramids of energy show that the producers store the highest quantity of energy and
the energy stored then decreases at each level in the food chain of the ecosystem.
Pyramids of numbers
A pyramid of numbers shows the total number of individual organisms at each level in
the food chain of an ecosystem.
A pyramid of numbers does not always have a regular pyramid shape because it does not
take into account the biomass of the organisms. An inverted pyramid of numbers can be
found in an ecosystem where the community contains a few producers with a very large
biomass that support a larger number of smaller consumers.
An inverted pyramid of numbers can also be found in an ecosystem where the
community contains parasites. There can be many more parasites than the hosts they
feed on because each individual parasite has a very small biomass.
Nitrogen in ecosystems
The chemical element nitrogen is important to living organisms because it is a
component of all the different amino acids that are used to make proteins.
The nitrogen cycle
Most nitrogen is found in the air as a gas. Most plants obtain nitrogen in the form of
nitrates from the soil and use these nitrates to produce amino acids. Animals obtain
nitrogen from the amino acids in the proteins they consume.
The movement of nitrogen between the air, soil compounds and compounds in the bodies
of living organisms is called the nitrogen cycle. Different groups of bacteria are very
important for keeping the cycle going so that nitrogen is always available in a form that
living organisms can use.
The stages of the nitrogen cycle
1. Nitrogen fixation: Legume plants such as peas, beans and clover contain
nitrogen-fixing bacteria. These bacteria live in swellings in the plant roots called
nodules. Nitrogen fixing bacteria convert nitrogen gas from the air into a form
that plants can use to make amino acids. Free-living nitrogen-fixing bacteria are
also found in the soil. When they die the nitrogen they have fixed into their
biomass is converted into ammonium.
2. Feeding: Animals consume plant protein, digest it using specific enzymes and
absorb the free amino acids.
3. Production of nitrogenous waste products: Animals cannot store excess amino
acids in their bodies. They convert them into waste products that contain
nitrogen and excrete them.
4. Decomposition: Decomposers (some free-living bacteria and fungi) break down
animal and plant proteins (from dead organisms) and nitrogenous waste products
to release energy. As a result of decomposition nitrogen is released into the soil
in the form of ammonium.
5. nitrification: A group of free-living soil bacteria called nitrifying bacteria
convert ammonium into nitrates in order to obtain energy.
6. Uptake of nitrates: Non-legume plants absorb nitrates from the soil into their
roots and use the nitrates to produce their amino acids to make proteins.
7. Denitrification: Another group of free-living soil bacteria called denitrifying
bacteria convert nitrates into nitrogen gas. Denitrifying bacteria are active
when the oxygen concentration of their environment is low, eg in waterlogged
soil.
Use of fertilisers
Humans can add nitrates to the soil artificially using chemical fertilisers. This can
increase the yield of crop plants grown in the soil.
Competition in ecosystems
Competition will occur between organisms in an ecosystem when their niches overlap,
they both try to use the same resource and the resource is in short supply.
Animals compete for food, water and space to live. Plants compete for light, water,
minerals and root space.
Interspecific competition occurs between individuals of different species.
Intraspecific competition occurs between individuals of the same species.
Intraspecific competition is usually more intense than interspecific competition
because the individuals have the same niche so are competing for exactly the same
resources.
Individuals that are better competitors will have a greater chance of surviving to
reproduce and pass on their genes.
Sampling techniques
A quadrat
It is impossible to count all the plants in a habitat, so a sample is taken. A technique
that is often used to sample plants is a quadrat. It marks off an exact area so that the
plants in that area can be identified and counted.
Facts about quadrats

Quadrats should be placed randomly so that a representative sample is taken.

Many quadrats should be placed so that a representative sample is taken.

Quadrats may also be used for slow moving animals such as snails/slugs.
There are some limitations of using a quadrat. Human judgement can be an issue when
using a quadrat. For example, some plants may be partially inside/outside a quadrat so
there are basic rules that scientists follow that reduce the chance of human judgement
affecting results.
Sampling animals
It is impossible to find and count all the animals in an area. You can get an idea of the
variety and number by taking a sample.
Pitfall traps are often used to sample the small invertebrates living on the ground. You
are likely to trap beetles and other insects, as well as spiders and slugs.
How to set up pitfall traps properly:

The top of the container should be level with the soil surface.

Cover the trap with a stone or piece of wood to keep out the rain, to make it
dark and to stop birds eating your catch.

The traps must be checked often to avoid the animals escaping or being eaten
before they are counted.

As with most methods a large number of traps makes results more reliable and
minimises the effects of unusual results.
Abiotic factors
Abiotic factors are non-living variables that can influence where organisms can live.
Examples of abiotic factors include:




light intensity
temperature
soil pH
soil moisture
Measuring light intensity
Light meters can be used to measure light intensity. The meter is held at the soil
surface and pointed in the direction of the maximum light intensity, and then the meter
is read.
Errors can be made when measuring light intensity by accidentally shading the light
meter. The reliability of the results can be checked by taking many samples.
Measuring the pH and moisture of the soil
Soil moisture and soil pH meters are also available. Both are used by simply pushing the
probe into the soil and reading the meter.
Errors can be made when measuring abiotic factors when the soil moisture or pH meter
probes are not cleaned between readings. The reliability of the results can be checked
by taking many samples.
Measuring temperature
The temperature of the air can be measured with a thermometer. The temperature of
the soil can be measured with a temperature probe.
Identifying organisms
Identifying organisms can be challenging, particularly when there could be hundreds of
different species within any one sample. Scientists use paired-statement keys to make
it easier to identify organisms that are sampled.
Paired-statement keys are set out as a list and you simply start at number one on the
list and work your way through until you reach the name of the organism.
Below is a simple paired statement key for garden invertebrates. Pick one of the
organisms and work your way through the flow chart to identify.
Impact of the increasing human population
Like all living things, humans exploit their surroundings for resources. Before the
beginning of agriculture about 10,000 years ago, small groups of humans wandered
across large areas, hunting and gathering just enough food to stay alive. Population
numbers were kept low because of the difficulty of finding food.
Population growth
The development of agriculture led to a population explosion that has accelerated
enormously during the past 500 years. Unlike other species, humans can adapt to and
survive in almost all terrestrial environments on the planet.
Human population growth over the past 10,000 years
The graph shows that the human population is growing. This is because the birth rate is
much greater than the death rate.
In the last fifty years, the population of humans on the planet has increased from 3
billion to 7 billion. This increase is more rapid than at any other point in the history of
our species.
The human population is predicted to reach around 10 billion by 2050. As the number of
people increases, the yield of food from farming will also need to increase or
malnutrition and starvation will occur.
The yield of crop plants per unit area of farmed land can be increased by intensive
farming methods, such as the application of chemical fertiliser and the widespread use
of pesticides. These methods have the advantage of increasing the quantity of food
that can be produced, but can also have an adverse effect on the environment.
Fertiliser
Chemical fertilisers are added to the soil to increase the quantity of nutrients such as
nitrates and phosphates available to plants.
Fertilisers can be lost from the soil by the process of leaching. Water moving through
the soil can dissolve the compounds in fertilisers and transfer them to lakes and rivers.
As the nutrient concentration of the freshwater increases, more algae are able to grow
and their population increases rapidly. This is called an algal bloom.
The breakdown of dead algae by decomposers can greatly reduce the concentration of
oxygen dissolved in the water. This can result in the death of fresh water animals and
plants.
Pesticides
Chemical pesticides are substances that are sprayed onto crops to kill organisms that
can reduce plant growth such as weeds, insects and fungi. Weeds compete with the
crop plants for resources, fungi can cause plant diseases and insects may consume the
plants and damage them.
Pesticides can have adverse effects on the environment if they are not biodegradable
as they can accumulate in the bodies of organisms over time.
Because the animals tend to eat lots of organisms from the level below in food chains,
the concentration of pesticide in the bodies of organisms increases at higher levels of
food chains.
This can result in the toxicity of the pollutant reaching fatal levels in the organisms at
the top of the food chain.
An example of a non-biodegradable, or persistent, pollutant is the insecticide DDT
which was used in the 1940s to kill mosquitoes. It accumulates in body tissues because
it is not very soluble in water so cannot be excreted.
The concentration of DDT in producers is very low, but increases greatly at higher
levels in the food chain. This can cause top-level consumers like ospreys to lay eggs
with much thinner shells that are more likely to break.
Indicator species
Indicator speciesare organisms that can tell us about the levels of pollution in an area
by their presence or absence.
Air pollution
The most common source of air pollution is the combustion of fossil fuels. This usually
happens in vehicle engines and power stations. Sulphur dioxide is released if the fuel
contains sulphur compounds.
This gas contributes to acid rain. Lichens can be used as air pollution indicators,
especially of the concentration of sulphur dioxide in the atmosphere.
Lichens are organisms that grow in exposed places such as rocks or tree bark. They
need to be very efficient at absorbing water and nutrients to grow there. Rainwater
contains just enough nutrients to keep them alive.
Air pollutants dissolved in rainwater, especially sulphur dioxide, can damage lichens and
prevent them from growing. This makes lichens natural indicators of air pollution.
For example:

Bushy lichens need really clean air.

Leafy lichens can survive a small amount of air pollution.

Crusty lichens can survive in more polluted air.
In places where no lichens are growing it is often a sign that the air is heavily polluted
with sulphur dioxide.
Water pollution
Water pollution is caused by the discharge of harmful substances such as untreated
sewage into rivers, lakes and seas.
Many aquatic invertebrate animals cannot survive in water that has been polluted and
resulted in a lowering of the oxygen concentration, so their presence or absence
indicates the extent to which a body of water is polluted.
Level of water pollution Indicator species
Clean
Mayfly larva
Low
Freshwater shrimp
High
Water louse
Very high
Rat-tailed maggot, sludge worm
The indicator species showing higher levels of pollution, such as the water louse, may
also be present in cleaner water. It is the absence of species that gives us information
about how polluted the water is.
Biological control
Biological control is an alternative to using pesticides and involves using one species or
biological agent to control the population size of another species. The organism used is
called a biological control agent.
By releasing a natural predator into the crop growing area, the number of pests can be
reduced. This can have unforeseen consequences as the numbers of different
organisms in the food web are changed. There have been examples of the predator
becoming a more serious pest than the original problem.
An example of biological control is the reduction in number of the Australian prickly
pear cactus (Opuntia). This cactus was introduced to Australia in 1840 and quickly
spread over large areas of land, making it unsuitable for farming.
Cactoblastis cactorum caterpillars on a leaf
To reduce the population size of cacti the moth Cactoblastis cactorum was introduced
from Argentina in 1925. The caterpillars of the moth consumed the cacti, and over a
period of a few years greatly reduced their numbers.
Genetically modified (GM) crops
Genetically modified (GM) crop plants have received genes from other organisms in
order to improve their characteristics. Genetic modification of crop plants may help to
reduce the impact of intensive farming methods on the environment.
Scientists have added genes from bacteria into the crop plants maize and potatoes
that make them toxic to insects. This reduces the need to use large quantities of
insecticides.
In the future it may be possible to introduce genes to crop plants that allow them to
use nitrogen fertilisers more efficiently, reducing the need to use large quantities of
fertilisers.